The use of a metallized plastic strip embedded within currency paper as a security thread for counterfeit deterrence is described within U.S. Pat. Nos. 4,652,015 and 4,761,205. The security thread is virtually undetected under reflected light and legible under transmitted light to verify its presence. In commercial situations where verification of currency bills is required, the receiver of the currency bill must subject the currency to a relatively intense light source to read the security thread under transmitted light. With large queues of customers at a bank or supermarket, as well as in places of low level illumination such as bars and restaurants it is difficult to visually inspect the corresponding large number of currency bills. It would be advantageous therefore to have some means of automatically determining the presence of the requisite security thread and confirming authenticity to the teller or cashier.
U.S. Pat. No. 4,524,276 entitled "Apparatus for Detecting a Security Thread Embedded in a Paper-Like Material" describes an infrared radiation source- and two infrared radiation detectors used to determine whether or not a security thread is embedded in the paper-like material and also to determine what the detected security material is made of.
Countries outside of the United States that employ plastic or metal security threads embedded in their paper currency, require that the presence of such security threads be ascertained under transmitted light such as described in the aforementioned U.S. Pat. No. 4,524,276. In accordance with the United States requirement that the currency security thread be detected under transmitted light and not seen under reflected light, both reflective and transmissive determinations are made for complete verification of the currency.
U.S. Pat. No. 4,980,569 describes a security paper verification device wherein optical means are arranged on opposing surfaces of the currency to determine the absence of any device on the surface of the currency paper while detecting the presence of the device within the currency. This is to prevent attaching counterfeit security threads to the outside surface of the currency paper to replicate genuine currency.
U.S. Pat. No. 5,151,607 entitled "Currency Verification Device" describes the combination of optical means with inductive or capacitive sensors for verifying the presence of the security thread in currency paper.
U.S. Pat. No. 5,260,582 entitled "Security Paper Verification Device" describes optical, magnetic and capacitive sensors used in combination to determine currency authenticity. The dark inks and dyes used in printing U.S. federal reserve notes could provide difficult indication of a metallized security thread when such optical sensors are used, per se.
U.S. Pat. No. 5,260,582 entitled "Security Paper Verification Device" describes an optical array arranged on both sides of a currency-receiving slot to determine whether the requisite security thread is present within the paper or on either surface. The device includes a microprocessor for calibration of the optical arrays.
U.S. patent application Ser. No. 115,775 filed Sep. 3, 1993 entitled "Security Paper Verification Device" describes an optical array directed on one side of proffered currency paper to determine whether the requisite security thread is present within the paper or on the outer surface. The device includes a microprocessor for calibration of the optical arrays. When moving the currency by the photodiodes or laser diodes used within the optical scanners that "read" the currency indicia, or when moving the scanners by the stationary currency paper, some definition is lost in the focus of the photodiodes or laser diodes on the currency paper surface requiring that the scanners be moved at a uniform speed across the paper surface and within a predetermined plane to prevent blurring of the currency image.
It would be advantageous to more rapidly scan the currency indicia in the least amount of time without having to carefully control the focal distance between the scanners and the top surface of the currency paper.
Accordingly, one purpose of the invention is to describe an optical system and a correction algorithm that automatically compensates for image blurring caused by the motion of the scanners or the motion of the currency paper.